1. Field of the Invention
The present invention relates to a receiver, a transmitter and a communication control program which communicate contents for which copyright protection is necessary via a wireless network.
2. Related Art
Products called as digital information consumer electronics have increased. It is predicted that these products will be widespread according to the beginning of contents distribution on the Internet. In a category of the digital information consumer electronics, there are included various kinds of products which deal with digital data and digital contents such as a mobile audio player, a digital broadcasting TV, a set top box and a hard disk recorder.
One of problems to be taken into consideration in accordance with spread of these products is copyright protection of contents. Digital data has an advantage in that quality does not deteriorate even if the contents are copied, but has a disadvantage in that illegal copy is easily conducted and copyright protection becomes insufficient.
Because of this, IEEE1394, which is a digital network connecting between digital AV equipments, has an authentication & key exchange function and a data encryption function.
Here, it is assumed to transmit AV data that copyright protection is necessary from a certain transmitter to a receiver. In this case, we have to pay attention that copyright protection relates to transmission of AV data only in a range that each individual person (or in a case of extended interpretation, each family member) enjoys. Transmission and reception of AV data between different persons should not be permitted, as long as permission of a contents provider is not obtained.
As one solution of protecting copyright on a network, there has been known DTCP (Digital Transmission Content Protection).
The DTCP is a copyright protection scheme which has become a de facto standard in IEEE1394, USB, IP and so on. The DTCP performs authentication & key exchange processing between the transmitter and the receiver, to transmit the encrypted AV data (see http://www.dtla.com).
Generally, in order to protect copyright in a transmission system, the AV data is transmitted by the following processing procedure. A command for transmitting and receiving AV data is issued between the transmitter and the receiver. For example, the receiver issues a reproduction command as one of AV control commands to the transmitter.
Subsequently, transmission of AV data from the transmitter to the receiver is begun after AV data is encrypted to protect copyright. Before or after the transmission of AV data, authentication & key exchange processing is performed to protect copyright between the transmitter and the receiver.
When the authentication & key exchange processing is successful, the transmitter and the receiver can share an encryption key of AV data, or calculate the encryption key by themselves. The receiver decrypts the encrypted AV data to reproduce the decrypted AV data.
Procedure of the DTCP is standardized as “DTCP over Bluetooth” even in Bluetooth included in a close range wireless standard. By using the DTCP, it is possible to securely transmit AV data that copyright protection is necessary.
However, progress of recent Internet technology causes new problems. In the Internet, it is possible to transmit and receive arbitrary data between users away an arbitrary distance.
Therefore, it is possible to easily transmit data between Japan and the United States at low cost.
It is possible to encrypt AV data that copyright protection is necessary for each packet. Also it is technically possible to transmit the encrypted packet in a capsulated state on Internet. Accordingly, there is a likelihood that a packet of AV data transmitted by the Bluetooth device which can communicate at only a close range may be converted into an internet capsule to transmit it to a distant location. In this case, if both of the receiver for receiving the internet packet and the Bluetooth device at sending side support the DTCP function, the authentication & key exchange processing is established between both of the receiver and the Bluetooth device, and it is possible to record and reproduce the AV data.
The “DTCP over IP” which prescribes procedure of copyright protection using the internet protocol (IP) adopts an RTT (Round Trip Time) as a technique for performing inside-home communication. More specifically, before or after the authentication & key exchange processing for copyright protection, or along the way of the authentication & key exchange process, the RTT is measured. The RTT is a time period from a time when a specific packet is sent from the transmitter to the receiver, until a time when the specific packet is sent back from the receiver to the transmitter. At this time, the receiver which receives the specific packet may transmit the specific packet as it is, or may include new data in the received specific packet. In this way, if the transmitter can correctly acknowledge a relationship between the transmitted original packet and the received packet, there is no particular limitation to a format of the packet sent back by the receiver. The transmitter measures the RTT, and if the measured value is equal to or less than a constant value, the transmitter can determine that the receiver is located in close range, i.e. the receiver is located inside the home. Therefore, it is possible to prevent transmission of AV data at a long distance.